1. Field of the Invention
The present invention relates to a printing device for printing bar codes that can be read automatically from all directions by means of an optical reader. More particularly, the present invention relates to improvements in a bar code printing device which is used with a portable label printing and applying machine or a table type automatic label printing machine for printing labels or tags with bar codes. See, for example, the following U.S. Patents and Applications of the inventor hereof: Ser. No. 158,320, filed Jun 10, 1980; U.S. Pat. Nos. 4,084,507; 4,018,157; and also U.S. Pat. Nos. 4,155,302; 4,149,460 and 4,055,118.
2. Description of the Prior Art
In large supermarkets, recently, the POS (Point-Of-Sale) system using a computer has been put into practice. In one standard type of POS bar code, as shown in FIG. 4 of the accompanying drawings, each numeral is composed of one character, comprising two dark bars having a length of 14 mm and two light spaces arranged parallel to and next to each other. The character thus composed has a width of 2.31 mm. A tolerance of .+-.0.101 mm is required.
A printing device for printing the bar code includes a plurality of bar code type rings, each of which includes a plurality of bar code types (representing the various numerals). Each bar code type is composed of the dark bars and light spaces having the standard sizes corresponding to each numeral. Even if the bar code type rings are accurately made, it is a problem to ensure the necessary accuracy in the size of the gaps between any two adjacent type rings arranged in alignment with one another. More specifically, since the width of each light space located at the end of each bar code symbol is significant in determining the value of the symbol, formation of extraneous gaps between the type rings will result in the optical reader reading the numeral incorrectly. If, on the other hand, the bar code type rings are fixed close together, it is impossible to selectively turn them independently of each other, as is necessary in setting the value of the number which is to be printed.
There has been proposed and used a bar code printing device of the type in which gaps are formed between the type rings in order to allow independent rotation thereof, thereby to facilitate the selection of the bar code symbols to be printed. The type rings are brought close together during printing operations, eliminating the gaps, so that widthwise printing errors of the type described above can be minimized.
During the use of the above described known device, however, it frequently occurs that the operator omits bringing the type rings close together after the type selecting operation and preparatory to the type printing operation, so that the printing operation is carried out with undesirably wide gaps between the type rings. The labels or tags printed while the printing device is in this condition are useless. In the event, moreover, that these labels or tags are erroneously applied to commodities, the result is that the optical reader fails to read out the correct information, which creates serious problems in the use of the POS system.